Serial content addressable memory

ABSTRACT

A technique is presented for implementing a content addressable memory (CAM) function using traditional memory, where the input data is serially loaded into a serial CAM. Various additions, which allow for predicting the result of a serial CAM access coincident with the completion of serially inputting the data are also presented.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/361,947, filed Feb. 23, 2006, which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention pertains to serially addressing a traditional highdensity semiconductor memory core in a pipe-lined fashion to obtain theequivalent function of a content addressable memory with less logic.

BACKGROUND OF THE INVENTION

Numerous examples of content addressable memories (CAMs) exist includingU.S. Pat. No. 6,999,331, granted Feb. 14, 2006 to Huang; U.S. Pat. No.6,389,579, granted May 14, 2002 to Phillips et al.; and U.S. patentapplication Ser. No. 10/609,756, filed Jun. 20, 2003, by Yen et al. Theyall refer to a bit of memory as taking from 8 to 16 transistorsdepending if the content addressable memory is binary, as shown in FIG.1 a, or ternary, as shown in FIG. 1 b, and while the demand for fastcontent addressable memory has grown with the need for addresstranslation in digital communications, the size of the memory haslimited the product offerings to under a megabit per chip, while DRAM isapproaching a gigabit per chip. At the same time, communications hasbecome increasingly serial. The primary communications input to routersand switches is high-speed serial digital streams of data, much of whichis self-clocked. Traditionally the address portion of a packet of datais serially read into a chip by a SERDES, and loaded into a register.Thereafter the register's contents are transferred to the data inputs ofthe CAM, to get the port address for the transfer of subsequent datastream in the packet.

The latency of the switching operation is critical to the smoothoperation of switches and routers, but as can be seen, the contentaddressable memory operation doesn't begin until after the entireaddress has been captured. This latency also increases as the number ofchips to hold the content addressable data grows because the hitsbetween multiple chips must be resolved before the port address can bedetermined. Therefore, the ability to economically hold greater amountsof content addressable data would also reduce the latency.

Dynamic random access memory (DRAM) structures are the densest volatiledigital memory structures in semiconductor technology. Each bit requiresonly one transistor and a capacitor, as can be seen in FIG. 2 a.Unfortunately, these memories are often quite slow. Alternatively thereexists static random access memory (SRAM) structures that require 3 to 6transistors per bit, as can be seen in FIG. 2 b, but are much faster,and require less sense circuitry. Traditionally, both of these types ofmemories are organized with more word lines than bit lines, since allthe bits in a word are accessed at the same time.

SUMMARY OF THE INVENTION

This disclosure describes a way to serially access all the data within ahigh-density DRAM or SRAM digital memory in a pipelined fashion, for thepurposes of translating the inputted data, based on its content, into anassociated value. To accomplish this, the memory core is organized withmore bit lines than word lines, and the data in the memory is transposedrelative to a normal memory, such that each word line addresses one bitof all the words in the memory, hence we will refer to it as a bit line.Similarly, in a traditional memory the accessed word of data isoutputted on the bit lines, but since the data is transposed in ourmemory, we will refer to these bit lines as data lines.

Using the new nomenclature, N bits of input data is entered into theserial CAM in a serial fashion, one bit at a time. The CAM consists of Mwords of memory and M sets of comparison logic, one for each word ofmemory. As each input data bit enters the CAM, the corresponding bit ineach word is accessed and available on that word's data line. The inputdata bit is then simultaneously compared with its corresponding data bitfrom all the words in the memory. The results of these comparisons arestored such that at the end of N clock cycles only those words thatmatch all the input data bits entered are still set. The resulting Mbits are then encoded and optionally translated into a port address, ifone exists.

In this fashion, the CAM operation still takes only one memory accessafter all address bits are available, but the memory may be DRAM orSRAM, which is much denser than a traditional CAM and therefore morecost efficient.

The present disclosure presents both a Binary serial CAM and a mixedBinary/Ternary CAM logic, both of which work using the above technique.

In addition, since the addressing sequence is the same for every CAMoperation, pre-fetch of the data is possible, which can, in some cases,with the appropriate look-ahead logic, completely eliminate the delayfrom the acquisition of the last bit of input data to the output of anappropriate translated address

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in connection with the attacheddrawings, in which:

FIGS. 1 a and 1 b are diagrams of binary and ternary CAM bits,

FIGS. 2 a and 2 b are diagrams of a DRAM bit and an SRAM bit,

FIG. 3 is a diagram of an embodiment of a serial CAM,

FIG. 4 is a diagram of one embodiment of the logic in each word of theserial CAM,

FIG. 5 is another embodiment of the logic in each word of the serialCAM,

FIGS. 6 a and 6 b are diagrams of Flip-flops,

FIGS. 7 a and 7 b are diagrams of encode logic,

FIG. 8 is another embodiment of the logic in each word of the serialCAM,

FIG. 9 is another diagram of encode logic with look-ahead,

FIG. 10 is a diagram of a serial CAM with parallel write capability,

FIG. 11 is a diagram of an SRAM bit for a serial CAM with parallel writecapability, and

FIG. 12 is a diagram of compare logic for the serial CAM in FIG. 10.

DESCRIPTION OF VARIOUS EMBODIMENTS

The present invention is now described with reference to FIGS. 1-12, itbeing appreciated that the figures illustrate the subjects matter andmay not be to scale or to measure.

A preferred embodiment of the present invention is a serial contentaddressable memory 30, consisting of a memory core 31, circular bitaddressing logic 32 serial compare logic 33 and encode logic 34 as shownin FIG. 3. It is further contemplated that the memory core may becomposed of either DRAM or SRAM cells. Each bit in the memory core 30 issuccessively addressed by clocking the circular bit addressing logic 32,which consists of a circular shift register comprised of N+1 bits withonly one bit set. Each of N bits drives one of the bit lines to thememory core 31. The N+1 bit drives a reset signal 35, which clears thecomparison bits to begin a new operation. To perform a contentaddressable memory operation, the input data is placed, one bit at atime, on the serial data in line 36, clocked into a single bit latch orflip-flop 41 when the content addressable memory is clocked. On the N+1clock, after the N bits of input data have been entered, the resultsfrom the encode logic 34 are available on the result lines 37. To adddata into the memory core, the write line 38 is set high, and N bits ofinput data are written into the next available word of memory, onsuccessive clock cycles. One or more, words of memory may be cleared byinputting an exact match of the word or words to be cleared and settingthe clear line 38 high, which clears all the words that match theinputted data.

Reference is now made to FIG. 4, one embodiment of the compare logicassociated with a single word of memory. Prior to loading the address,each compare latch or flip-flop 42 for all the words in memory issynchronously set to logic level 1 by the reset signal 35. At thebeginning of the compare operation, when the memory is clocked, thefirst bit of data from the word in memory is captured in a latch orflip-flop 42. On the next clock cycle this data is compared with thedata in value by an XNOR gate 44, and the results are gated by the stateof the compare flip-flop 42 and the address flip-flop or latch 46, whichis 1 if this word has contents to compare. On this and each successiveclock cycle, the compare flip-flop 42 continues to capture a logic 1level as long as the word has contents (the address flip-flop is set toa logic level 1) and the output of the data in flip-flop 42 is the sameas the bit from the memory flip-flop 43. After comparing all N bits ofthe serial data in 36 with all the bits in the memory core 31 thecompare flip-flops 42 that still have a logic level 1, signify a match.

When writing a new word into memory, the write signal 36 is set high,which drives the enable signal 48, for the first word high. The AND gate47, gates the enable signal 48 low after passing the first unused word.The AND gate 49 selects the data from the memory flip-flop 43 for allother words except the first unused word, where the data from the datain flip-flop 41 is selected. The AND gate 50 ensures the addressflip-flop 46 is set after writing the data into that word of memory, onthe next cycle reset is set high.

To delete a word from memory, the clear signal 39 is set high on thesame cycle reset is set high. The AND gate 51 clears the addressflip-flop or latch 46 of the words whose compare signals 57 are stillset high. The words that matched data inputted on the serial data inline 36, are cleared. It also should be noted that the memory may haveseparate lines for reading and writing 52 as shown here, oralternatively a single data line 56 for both reading and writing data tomemory, as shown in FIG. 5.

As can be seen by in FIG. 4 and description above, this logic ensuresloading, erasing and binary comparing of data within one clock cyclefollowing the serial loading of input data.

Some CAMs have multiple bits per bit they compare. Three values areencoded in the two bits; zero, one and don't care. The “don't care”value always compares with the input data. This is traditionally calleda Ternary CAM.

In another embodiment of the present invention, a variably mixedBinary/Ternary CAM is presented. Furthermore the number of binaryentries in the memory and the number of ternary entries is solely afunction of what is written into the memory, since any entry may bedefined as binary and any pair of entries may be defined as ternary. Inaddition a binary entry may be converted into a ternary entry by addingits corresponding ternary data in the word following the binary entry.

Reference is now made to FIG. 5, another embodiment of the compare logicassociated with a single word of memory. In this embodiment of thecompare logic there exists two address flip-flops per word; one addressflip-flop 46 identifies the existence of normal data, and one ternaryflip-flop 55 identifies “don't care” ternary data. The logic 53 ensuresonly one of the two flip-flops 46 and 55 is set after writing a word ofdata into the memory. This is done during the reset cycle following awrite operation by setting the Binary-Ternary signal 40 either high tosignify normal data or low to signify ternary “don't care” data. In thisembodiment a bit of “don't care” data is signified by a logic level 1 ina word whose ternary flip-flop 55 is set high. The ternary data is notcompared with the values on the serial data in flip-flop 40, rather twoadditional gates 54 combine the values from the memory flip-flop 43 andthe ternary flip-flop 55 and use it to select or ignore the bit compareof the previous word. In this way logic level 1s in a word of ternarydata that follows a word of addressed data act as “don't care” bits whencomparing the addressed data with the serial data in. When clearingwords from memory, both the address flip-flop 46 of the compared wordand the ternary flip-flop 55 of the next word are cleared. In thisfashion any amount of binary or ternary data may be loaded into thememory 31, and varied as needed. Furthermore if the word following aword of binary data is unused or cleared, it may be loaded with ternarydata, and subsequently all comparisons of the inputted data with thebinary data from that word will use the following word as ternary “don'tcare” data in the comparison.

Reference is now made to FIG. 6 a, a diagram of a Flip-flop. The symbol60 is equivalent to the diagram 61. The clock 62 is implicit. Thissymbol is used for Flip-flops 42, 43 and 80 in FIGS. 3, 4, 5 and 8.

Reference is now made to FIG. 6 b, another diagram of a Flip-flop. Thesymbol 63 is equivalent to the diagram 64, which includes a NAND gate 65to synchronously clears the Flip-flop 67, a multiplexor 66, which eithersynchronously loads a 1 into the Flip-flop 67 when the select input 68is 1 or re-circulates the data in the Flip-flop when the select input 68is 0. This symbol is used for Flip-flops 46 and 55 in FIGS. 4, 5 and 8,and Flip-flops 121 and 123 in FIG. 12.

Reference is now made to FIG. 7 a, a diagram of an embodiment of theencode logic for a single word. This version of compare logic consistsof a multiplicity of transistors 70 whose gate inputs 71 are either tiedto ground in such a way that each compare line 57 through an optionalbuffer 77 selects a unique combination of transistors. The data lines 72are typically pre-charged to a high level such that when a compareoccurs, the transistors connected to the buffered compare line 78 pulltheir data lines 72 low, to output the address of the matching word. Theinputs 73 to the control logic 74 are tied to ground before the firstword. The match line 75 is set high when one or more compare lines 57are high, and the −valid line 76 is set high when two or more comparelines 57 are high. The data lines 72 and control lines 75 and 76 formthe set of result lines 37 shown in FIG. 3.

It is further contemplated that different control logic may gate thecompare line 57 at the buffer 77 to select just the first compare linethat is high, and reset the compare flip-flops of selected words onsuccessive cycles until −valid is low.

Reference is now made to FIG. 7 b, another diagram of encode logic for aword of memory. Alternatively the compare line 57 may select a word ofmemory 79, which may also be loaded following the loading of thecorresponding word of the memory core 31 in FIG. 3.

In another embodiment of the present invention, look ahead may beemployed in the comparison process to occasionally finish the comparisonprior to comparing the last serial data in bits with their correspondingbits of memory.

Reference is made to FIG. 8, another embodiment of the compare logic foreach word of the serial CAM. In this case there is an additionalflip-flop stage 80, such that at least one clock cycle ahead of thefirst serial data in value, the first bit of every word of memory isloaded into the memory flip-flop 43. On the next clock, the first bit ofserial data in is compared with the first bit of memory data that istransferred to the additional flip-flop 80. At the same time the nextbit of data is available in the memory flip-flop 43. The value in thememory flip-flop 43 is gated 81 with the ternary flip-flop's value andset on the look-ahead line 83 for the previous word. The look-ahead lineis only set when the next bit is a ternary “don't care”.

References now made to FIG. 9, another diagram of encode logic, but withlook-ahead logic. In this example the look-ahead line 83 is gated with aglobal penultimate line 90 that is obtained from the circular bitaddressing logic 32 in FIG. 3, in a manner similar to the reset line 35,in FIG. 3, and is further gated with the compare line 57. The resultingcompare 91 drives the control logic 64. This has the effect of limitingthe compares to those words that compare up to the last bit and willcompare on the last bit because it is a “don't care”. If a match occurson this next to last cycle, the results may be captured on the sameclock that is capturing the last serial data in bit into the flip-flop40 in FIG. 3, which is equivalent to zero latency between the serialdata in and the results out.

In yet another embodiment of the present invention, the memory core mayhave orthogonal read and write operations, which allows for parallelloading of the CAM data, while continuing with serial reading of thedata. This is desirable since in most systems the data being loaded intothe memory is obtained, generated and stored in parallel form, whereasmost of the data being used in the CAM access, to obtain translatedresults, is transferred to the CAM serially.

Reference is now made to FIG. 10, a diagram of a serial CAM withparallel write capability. In this embodiment, the memory core 100includes a parallel data in port 101 for writing data into the memory.The address lines for writing the parallel data into the memory areperpendicular to the parallel data in lines and originate in the comparelogic module 102. The bit address lines originate in the circular bitaddressing logic 103, and select the bit to be read into the comparelogic module 102 for each word.

Reference is now made to FIG. 11, a diagram of an SRAM bit for a serialCAM with parallel write capability. It is relatively common in a twoport SRAM to make the inverter 110, which drives the read word line 111larger than the inverter 112, which must be over driven by the bit dataline 113, but in this case since the word lines are relatively shortcompared to the number of words, the optimal ratio may be larger than anormal SRAM. Similarly, it is relatively common to have a larger Nchannel transistor 114 for writing than the N channel transistor 115 forreading, but in this case the optimal ratio will also have to beadjusted for the different configuration of the memory, since thecapacitance of the word address line 116 is less important than the bitaddress line 117 capacitance in this configuration. What is unusual isthe orthogonal nature of the read versus write.

Reference is now made to FIG. 12, a diagram of compare logic for theserial CAM in FIG. 10. Just as in the previous versions of the comparelogic, the read word line 111 from the memory drives a memory flip-flop124, but in this case the enable logic 120 drives the word address line116 directly, selecting the first available word address to write into,regardless of whether a comparison is currently being done. The addressflip-flop 121 is enabled only after the reset line 108 is set high, butduring each write the alternate address flip-flop 123 is enabled to beloaded with a 1. Both the address flip-flop 121 and the alternateaddress flip-flop 123 control the next word's enable signal through anOR gate 124. As a result multiple writes can occur simultaneously withthe serial read of the CAM, but the new data will only be activated atthe beginning of the next CAM access.

It is contemplated that other parallel operations may be included inother versions of the serial CAM such as parallel write of the encodememory, if it exists, through data inputs 116 as shown in FIG. 10, orparallel operations to load binary and ternary data by combining thefeatures of the compare logic in FIG. 8 with the features of the comparelogic in FIG. 12. It is also contemplated that other look-aheadfunctions may be similarly generated, such as including a last bit valueand presenting the results if the selected words in a memory such asshown in FIG. 7 contain the same value. It is further contemplated thatstandard forms of addressing, or serial addressing may be used to loadand clear the memory cores 31 and any memory that exists in the encode34 shown in FIG. 3, or in the memory 100 and encode 104 shown in FIG.10. Furthermore it is contemplated that different combinations of logic,some of which employ fewer transistors, may be employed in producing thefunctions or functions similar to the functions described in the comparelogic 33 and the encode logic 34 in FIG. 3, or in the compare logic 102and the encode logic 104 in FIG. 10. Lastly it is contemplated thatvarious forms of non-volatile memory such as ROM, and PROM may be usedin place of the DRAM or SRAM in the memory core 31 and encode 34 in FIG.3, or memory core 100 and encode 104 in FIG. 10.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the present inventionincludes both combinations and sub-combinations of various featuresdescribed hereinabove as well as modifications and variations whichwould occur to persons skilled in the art upon reading the foregoingdescription and which are not in the prior art.

1. A content addressable memory comprising: a single serial data input;circular bit addressing logic; a memory core; compare logic for eachword of said standard memory; and encode logic; wherein serial data isserially loaded through said serial data input into said contentaddressable memory, said circular bit addressing logic successivelyreads successive bits of all words in said memory core, and said comparelogic successively compares said serial data with each bit of each usedword read from said memory core.
 2. A content addressable memory as inclaim 1, further comprising: a multiplicity of parallel data inputs;wherein parallel data is loaded through said parallel data inputs, andsaid parallel data is written into said memory core coincident withloading serial data.
 3. A content addressable memory as in claim 2,wherein said parallel data written into said memory core is marked asused following the loading of the last bit of said serial data.
 4. Acontent addressable memory as in claim 2, wherein said writing into saidmemory core and said reading from said memory core are orthogonal.